<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>flex</title>
    <style>
        .box {
            display: flex;
            width: 300px;
            height: 100px;
        }
        .box div{
            width: 100%;
            border: 1px solid #000;
        }

    </style>
</head>
<body>
    <div class="box">
        <div>1</div>
        <div>2</div>
        <div>3</div>
    </div>
</body>
</html>